The mechanisms which allows localization of proteins to the nucleus and the various organelles of the secretory pathway are under investigation. Using a temperature sensitive mutant of Vesicular Stomatitus Virus (VSV) it is possible to synchronize the intracellular transport of the single glycoprotein of the virus (G protein). The mutant G protein accumulates in the nuclear envelope (NE) and rough endoplasmic reticulum (RER) at 39 degrees C and only proceeds through the secretory pathway at 32 degrees C. We have demonstrated that the NE is not required for transport of G protein. Using an antisera which recognizes only the cytoplasmic domain of this transmembrane protein, the organelles to which it is routed may be purified by immunoaffinity isolation techniques. The large T antigen of SV-40 is a virally encoded protein which is synthesized in the cytoplasm yet is found almost exclusively in the nucleus of infected cells. The primary sequence requirements which allow this localization have recently been defined. Short peptides made up of this sequence have been used to generate antisera against the nuclear localization sequence of the large T antigen and are being used to attempt to identify the molecules involved in translocation of the protein to the nucleus.